In a variety of transmission systems, a weighted and delayed equalizing filter circuit is frequently employed as a technological means for electrically compensating for waveform distortions that are generated due to dispersion or loss in a transmission medium.
FIG. 1 is a circuit block diagram of a conventional weighted and delayed equalizing filter circuit. The circuit in question is often called a transversal filter, a finite impulse response (FIR) digital filter, or a feed forward equalizer.
This type of circuit has been described in, for example, the literature (“High-Speed Generalized Distributed Amplifier-Based Transversal-Filter Topology for Optical Communication Systems,” A. Borjak, et al., IEEE Trans. Microwave Theory Tech., Vol. 45, No. 8, pp. 1453-1457).
As shown in FIG. 1, such a transversal filter consists of n-delay devices 1105 and n-weighting circuits 1106 wherein n is an integer equal to or greater than 2. A signal inputted to the transversal filter is set to a desired delay by the respective delay devices 1105 to be outputted as the respective output signals. Then, desired weight values (filter coefficient) are given by the respective weighting circuits 1106. Thereafter, the signals from weighting circuits are added together to be eventually outputted. By performing this kind of signal processing, waveform equalizing is realized by directly compensating for a waveform distortion in an electric signal as well as by carrying out photoelectric conversion of an optical dispersion waveform by a photodiode or the like, followed by correction in optical communications.
FIG. 2 is a circuit block diagram illustrating an exemplified configuration of a transversal filter circuit. A conventional transversal filter circuit of this type has been described in, for example, the literature (“Differential 4-tap and 7-tap Transverse Filters in SiGe for 10 Gb/s Multimode Fiber Optic Equalization”, International Solid-State Circuit Conference (ISSCC) 2003, Preliminary Summary, Paper 10.4). As discussed in this literature, this transversal filter circuit is configured as follows using a distribution-type circuit in order to implement the above waveform equalizing function.
In such a transversal filter circuit, a 50-ohm matching transmission line connected in cascade to the input and output terminals forms delay devices 407, amplifiers each attached with a gain-adjusting terminal for setting a filter coefficient, which are installed at portions corresponding to respective amplifying stages, form weighting circuits 408, and a 50-ohm load resistance connected to output-side delay devices 409 forms an adder, to thereby constitute the transversal filter circuit. According to this circuit configuration, an input signal is given delays and weighting values so that desired waveform equalization can be achieved.